1. Field of the Invention
The present invention relates to a mold assembly used in injection molding, compression molding and like molding processes for molding thermoplastic resins, thermosetting resins and like synthetic resins into a molded product, more particularly to a synthetic resin molding mold assembly, in which a cavity surface of a mold piece of the mold assembly is alternately heated and cooled in the molding processes.
2. Description of the Related Art
As a factor in the causation of a defect in the quality in of appearance of such a molded product, there is a so-called “weld line” problem, which comes up in the art.
As for the weld line, it tends to appear in the confluence area of streams of a plurality of streams of a molten resin that filled the mold cavity when such streams having upstream portions that differ in flow direction and have their leading end (i.e., downstream end) portions combined with each other inside the mold cavity. More specifically, when these leading stream end portions in streams of the molten resin are brought into contact with a cavity surface of the mold cavity in the assembly, a surface of the molten resin is cooled down and a solidified resin layer is formed in each of the leading end portions of these streams of molten resin inside the mold cavity. As a result of such solidified resin layer, it becomes impossible for this resin layer to closely replicate a surface of the mold cavity in the assembly even when the molten resin is completely filled in the mold cavity and retained therein under a predetermined retention pressure. A failure in closely replicating the surface of the mold cavity in the assembly leads to the formation of a tiny groove. The groove assumes a V-shaped form (in cross section) on the surface of the molded product and is called the “weld line” in the art.
For example, when a molded product 1 provided with a plurality of opening portions 2 (see FIG. 13) is molded by using a conventional type of mold assembly 3 (see FIGS. 11 and 12), the molten resin is supplied to cavity space 7 of the mold piece of the assembly 3 through its sprue 4, runner 5 and its gate 6, and filled in the cavity space 7. The molten resin thus supplied is divided into a plurality of streams in the mold cavity of the assembly 3 by means of a plurality of core portions 8 (see FIG. 11) of the mold assembly 3. Each of the core portions 8 is adapted to form each of the opening portions 2 of the molded product 1; and, the streams of molten resin thus supplied and divided are combined with each other in a downstream side of each of these core portions 8. As a result, a plurality of weld lines 9 are produced in the downstream sides of the ss core portions 8 in the molded product, as shown in FIG. 13.
The molded product is made of polycarbonate (transparent resins) and has: a length of 100 mm; a width of 50 mm; and, a thickness of 1.2 mm, that is, a size of 100 mm length×50 mm width×1.2 mm thickness. The gate of the mold is constructed of a single side gate only.
In order to prevent the weld line from occurring in the molded product, it is necessary to heat the mold assembly to a relatively higher temperature when the molten resin is fills the mold cavity of the assembly. After molten resin fills in the mold cavity of the assembly, a predetermined retention pressure is applied to the mold assembly. However, heating the mold assembly to such a relatively higher temperature will be followed by a relatively longer period of cooling time of the mold assembly thus heated. This results in a longer molding cycle time since it is difficult to sufficiently cool down the molded product in a short period of time. This results in a molded product still not sufficiently cooled down, and difficult to smoothly separate it from the mold cavity of the assembly. In view of the above-mentioned difficulty, various systems described before have theretofore been proposed. These relate to a heating process of the cavity surface of the mold assembly which is carried out only when the molten resin is filled in the mold assembly.
A) Hot Water/Cold Water Switching System:
In this system “A”, hot water and cold water are alternately supplied to a mold temperature controlling water pipe to control a temperature of the mold assembly.
This system is advantageous because it may use an ordinary type of mold assembly and does not require any large investment for installations appurtenant to the mold assembly.
On the other hand, this system is disadvantageous since the mold temperature controlling water pipe is positioned apart from the mold surface, and since a response in controlling or increasing/decreasing a temperature of the mold assembly is not sharp; (an upper limit in temperature of the mold surface is set at a temperature of 160° C.), this system is not adequate for a resin molding process having a high glass transition temperature.
B) Cold and Heat Switching System (Steam Heating System):
In this system “B”, steam and cold water are alternately supplied to a fluid passage for controlling a temperature of the mold assembly.
This system “B” is advantageous in that the rate of an increase in temperature of a mold cavity surface employed is larger than that of an increase in temperature of a mold cavity surface employing the hot water/cold water switching system (A). Further, the system (B) is advantageous in that the system (B) construction, through which a heating and a cooling medium for respectively heating and cooling the mold assembly flows, is closely arranged with a resultant reduction in temperature difference of the mold cavity surface.
On the other hand, this system (B) is disadvantageous because the system (B) restricts an upper limit in temperature of the mold cavity surface up to a temperature of 155° C. and therefore, synthetic resins which are to be used in the mold assembly employing this system (B) are strictly restricted. A very expensive production line provided with a boiler, a medium switching device and like equipment is required in the system (B). Also, any mold assembly used in the system (B) tends to rust in use even when an appropriate sealing agent is used to seal the mold assembly. Since a mold insert of a split type is used and supported by a rib in the mold assembly employing the system (B), it is necessary to increase the mold assembly thickness so as to keep its mechanical or physical strength. As a result, it is not possible for the mold assembly employing the system (B) to have its fluid passage in the vicinity of the mold cavity surface.
C) Mold Surface Insulated System:
In the mold assembly employing this system (C), a heat insulating thin film layer made of ceramic material is provided in the mold cavity surface. This arrangement impairs the heat conduction between the molten synthetic resin and mold parts or mold pieces of the assembly, so that the cooling and solidification process of a molten resin to be formed into a molded product is delayed.
This system (C) is advantageous in that does not require any introduction of new installations for production; since the heat insulating thin film layer is formed by using a surface coating technique, there is no need of directly machining any mold parts in the mold assembly employing the system (C).
On the other hand, this system (C) is disadvantageous since the rate of an increase in temperature of the mold surface is small in the mold assembly employing this system (C). A weld line when it occurs tends to remain and appear in the molded product. Furthermore, system (C) does not allow application of any additional processing work to an outer surface of a mold cavity of the mold assembly and, it is impossible to finely control the mold cavity surface in temperature.
D) High Frequency Induction Heating System:
In this system (D), a magnetic field is produced by using an electric current supplied to an induction coil disposed adjacent to the mold cavity surface, so that an electric current is induced in the mold cavity surface. The Joule heat produced increases the temperature of the mold cavity surface.
This system (D) is advantageous because it is possible for the mold assembly to have its mold cavity surface temperature to be sufficiently increased, i.e., it is possible to heat the mold cavity surface up to a temperature of 250° C. or greater; also the rate of increase in temperature of the mold cavity surface is large; and with in this system (D), since heating of the mold cavity surface is obtained using an extraneous means, processing or machining the mold piece is not necessary.
On the other hand, this system (D) is disadvantageous since it is not possible for a plurality of induction coils to heat a portion of the mold cavity surface disposed between adjacent induction coils, system (D) tends to suffer from large variations in temperature of the mold cavity surface. It is still not possible in the art to produce certain unusually shaped induction coils necessary to follow a corresponding shape of a mold cavity surface and, since heating of the synthetic resin in the molding process is not realized in the mold assembly employing this system (D), the resultant molding process takes a much longer period of processing time (i.e., much longer molding cycle time).
E) Radiant Heating System:
In system (E), the mold cavity surface is radiated with a halogen lamp when the mold assembly is opened in retrieving the molded product.
System (E) is advantageous since radiation of the mold cavity in the mold assembly is realized by an extraneous means, and it is not required to apply a processing or machining work to the mold assembly itself.
On the other hand, the system (E) is disadvantageous because an increase in temperature of the mold cavity surface takes a much longer period of radiation time. Also, when a mold cavity surface of the assembly varies in height, system (E) cannot meet such a mold cavity surface that varies in height.
F) Electrically Energizing System:
In this system (F), a mold assembly has its mold cavity surface coated with an insulating layer and the insulating layer is further coated with an electrically conductive layer. By directly supplying an electric current to an electrode of the mold assembly, heat is produced to increase temperature of the mold cavity surface temperature.
System (F) is advantageous because the rate of increase in temperature of the mold cavity surface is large; and it is possible to keep the mold cavity surface at a relatively higher temperature (i.e., a temperature of 250° C.). Also, there is substantially no need of processing or machining the mold assembly itself.
On the other hand, this system (F) is disadvantageous since an electric current tends to pass through the shortest possible path between electrodes, it is not possible for this system (F) to evenly heat the mold cavity surface.
G) Cartridge Heater System:
In this system (G), a cartridge heater is provided in the mold assembly to heat the assembly.
System (G) is advantageous as it is possible to keep the mold assembly at a relatively high temperature range and, the mold assembly employing this system (G) is installed in an easy manner.
On the other hand, system (G) is disadvantageous because a relatively longer period of time is required in changing a temperature of the mold assembly as it is generally impossible to control a temperature of the mold assembly during its molding cycle. Furthermore, when the mold cavity surface varies in height, it becomes impossible to evenly heat the mold cavity surface since the cartridge heater is not flexible in construction. Furthermore, to reduce a difference in heating effects in the mold cavity surface, it is necessary to increase the number of heaters.
As for the above-mentioned various systems, anyone of these systems has both its advantages and disadvantages. For example, in some systems, a flow of cooling medium passes through the mold assembly and in another system, a cartridge heater is inserted into the mold assembly. In all cases, it is necessary to form the mold assembly with a fluid passage adapted for such cooling medium; and to have an aperture adapted for receiving the cartridge heater therein. Limitations exist when using a drill to form an insertion aperture in an outer wall of the mold assembly, because it is useful only when a straight line fluid passage can be formed; and the mold assembly allows a straight line heater to be installed in the assembly. Due to the presence of such limitations, it is not possible for anyone of the above-mentioned systems to keep a constant distance between the mold cavity surface and heating means when the mold cavity surface has a three-dimensional concave-convex contour, and such a three dimensional concave-convex contour prevents the mold cavity surface from being evenly heated.
In order to avoid a problem derived from the above limitations when processing or forming the insertion aperture, it is possible to divide a mold insert of the assembly into a plurality of split-insert pieces, wherein the plurality of split-insert pieces may be assembled into a three-dimensional circuit. In this case, when a fluid is used as the medium, and in order to avoid any possible leakage of the medium or fluid itself, it is necessary to seal all the fluid passages in the mold assembly. It follows that any mold surface thus sealed cannot be used as a reception surface for supporting the mold insert such that the mold insert abuts on the reception surface. Due to this condition, the mold insert is not sufficiently supported in the mold assembly, which decreases the physical strength of the mold assembly. In the case where the split-insert pieces of the mold insert are used, another problem arises; namely, a distortion of the mold assembly appears in the mold piece when a mold cavity surface or inner surface is present up to a depth of 4 mm or is more than 4 mm measured from an outer surface of the mold piece.
On the other hand, in the case where the fluid is used to heat the mold assembly, a difference in temperature appears between an inlet opening and an outlet opening of the circuit of the mold assembly. Furthermore, another problem arises in the case where heaters are used to heat the mold assembly, as it is not possible to arrange these heaters in close relationship with each other, which results in a variation in temperature of the mold cavity surface.
When each of the fluid passages for receiving the medium or a space adapted for the heater insertion aperture has a width of 4 mm or more and is formed in a position having a depth of 4 mm or less measured from the mold cavity surface, an area of the mold cavity surface corresponding in position to the space is distorted under a molding pressure applied to the resin filled in the mold assembly. Due to such distortion, a surface of the molded product is impaired in luster in its appearance, which leads to a defect in appearance quality.
In order to avoid these problems, it is necessary to provide the fluid passage of the medium or to provide the heater insertion aperture having a depth of 4 mm or more measured from the mold cavity surface. On the other hand, the rate of an increase in temperature of the mold cavity surface depends on both the above-mentioned depth and a heat transfer rate of material of the mold piece provided with the cavity surface, and is therefore small.
When a weld line or defect of the molded product in appearance is removed from the molded product, it is possible to eliminate the decorative coating process of the molded product carrying the weld line. This leads to a considerable cost reduction in producing the molded product, and is therefore considered as a desirable object in the art.
As a means for accomplishing the above object, it is proposed in the art to increase a temperature of the mold cavity surface to a higher point than an ordinary, whereby the weld line has its groove reduced in depth. Furthermore, it is also well known in the art that when a temperature of the mold cavity surface is increased up to a predetermined point inherent in the resin or material of the molded product, any weld line disappears. On the other hand, when the temperature of the mold cavity surface is always kept at a higher point, a resultant molded product which is still not sufficiently solidified is ejected from the mold assembly. Such an ejected mold product is often deformed upon its ejection from the mold assembly. Consequently, to avoid this condition, it is necessary to keep the mold cavity surface at a higher temperature during the resin filling process for applying a mold retention pressure to the mold assembly, and lowering the temperature in the mold cavity surface to a point at which the molded product completes its solidification and is ejected from the mold assembly.
Due to this, it is necessary to alternately heat and cool the mold cavity surface in one cycle of the injection molding process of the molded product thus produced. Therefore, various types of the above-mentioned systems have been proposed in the art. However, any one of these systems having been proposed in the art have advantages and disadvantages. Under such circumstances, there is still no established and reliable technique in the art with respect to the instant subject matter.